JP 2002-349357 A describes a device that determines an abnormality in an exhaust gas recirculation system (hereinbelow termed “EGR system” (EGR being an abbreviation of Exhaust Gas Recirculation)) of a combustion engine. The EGR system has an EGR pipe arranged between an exhaust pipe and an intake pipe of the combustion engine. An EGR valve that opens and closes the EGR pipe is arranged on the EGR pipe.
A part of gas discharged to the exhaust pipe from the combustion engine is supplied to the intake pipe through the EGR pipe. A throttle portion is disposed at a connecting portion between the EGR pipe and the exhaust pipe. Thus, the throttle portion restricts gas flow rate in a case where the EGR pipe and the exhaust pipe are connected normally. On the other hand, in a case where the EGR pipe is detached from the exhaust pipe, the EGR pipe comes to be in a state of communicating with open air without intervention of the throttle portion. As a result, a pressure in the intake pipe changes between the case where the EGR pipe and the exhaust pipe are connected normally (that is, the case where the gas is supplied to the intake pipe through the throttle portion) and the case where the EGR pipe and the exhaust pipe are not connected normally (that is, the case where the air is supplied to the intake pipe without the intervention of the throttle portion). In the exhaust gas recirculation system of the combustion engine, a determination is made on whether the EGR pipe and the exhaust pipe are connected normally or not based on the change in the pressure in the intake pipe.